This invention relates to improvement of a heat transfer sheet (a heat-sensitive transfer sheet), and more particularly, to a heat transfer sheet capable of providing high quality printing even in the case of a transferable paper (i.e., a paper to be transferred) having a low surface smoothness and further capable of preventing any staining (e.g. scumming or smudging) caused by a hot melt ink composition.
When the output print of computers and word processors is printed by heat transfer systems, a heat transfer sheet comprising a hot melt ink layer provided on one surface of a film, as well as at least one thermal head are used. Prior art heat transfer sheets are those which are produced by using, as a base film, papers such as condenser paper and paraffin paper having a thickness of from 10 to 20 .mu.m, or films of plastics such as polyester and cellophane having a thickness of from 3 to 20 .mu.m, and coating on the base film described above a hot melt ink layer wherein pigments are incorporated into waxes. The heat transfer sheet is used in the form of a film or in rolled form in most cases.
In general, however, when heat transfer printing is carried out, a hot melt ink layer of a heat transfer sheet directly contacts with the surface of a transferable paper, and a time lag between the moving velocity of the heat transfer sheet and that of the transferable paper is liable to occur at the time of initiating and stopping the printing or moving to a new line. This is because staining occurs. Particularly, in high-speed printing the staining is liable to occur.
While the heat transfer system can be used to print to common papers, distinct printing is not necessarily carried out in all the common papers. It is possible to carry out maximum printing if the transferable papers are calendered wood-free papers or coated papers which exhibit a value of at least 100 seconds when the smoothness of the transferable papers is expressed in terms of Beckmann smoothness. Even in the case of wood-free papers having a value of the order of 50 seconds, sufficient printing quality can be obtained. However, when transferable papers having a low smoothness i.e., less than 50 seconds are used, the distinctiveness of printing is reduced. This is because in the case of papers having very uneven surfaces, an ink composition cannot entirely come into contact with papers under a thermal head-urging pressure and the uncontacted portions exhibit inferior transfer.
Further, the heat transfer system is slower in printing speed as compared with an impact system, and improvement is required. In order to carry out printing at a higher speed, the level of heat energy which is given to a thermal head must be increased. However, this tends to lead to bleeding of printing and to make the staining described above worse.
We have carried out studies in order to overcome the drawbacks and disadvantages described above. It is an object of the present invention to provide a heat transfer sheet wherein no staining is generated even if high speed heat transfer is carried out and wherein distinct printing can be obtained even in the case of transferable papers having a low surface smoothness.